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Hot air rework station pumps - food for thought ?
Fraser:
I recently bought a barely used JBC JT6040 hot air station and also found a heavily used older JT6040 as a potential spares donor for the future. The JT6040 is a very nice piece of equipment but it’s design has lead to me looking at the different types of air pump/compressor used in hot air stations.
The newer model of JT6040 uses a Regenerative air compressor driven by a 240V Universal brushed motor. The Compressor motor RPM is controlled via a pretty standard ac motor speed controller. The older version of the JT6040 uses a different air pump design. The compressor is long and relatively small diameter and appears to be a multi stage turbine of some sort. The motor used to drive the compressor stages is a brushed DC type that is mounted in the air path within the compressor casing. This compressor is surprisingly noisy in operation and sounds like it has some form of gearbox within it. A small fan is visible at the rear of the tubular housing but this alone would not produce the required performance. I will add pictures of these two JBC air pumps to this post later.
Seeing these two different forms of air compressor got me thinking about the different technologies I have seen in my hot air rework stations. These are the ones that I have seen used in this application (including the JBC units mentioned above):
1. Dual diaphragm pump - similar in design to aquarium pumps and uses a pair of bellows connected to an inductively driven linear movement armature.
2. ‘Turbine’ blower - Pace have used what appears to be a variant of a centrifugal fan but they describe it as a “turbine”. It bears no resemblance to the multi stage turbine compressors used in aero engines though !
3. Centrifugal fan - Used in many recent models of hot air rework station. Mounted either in the station base unit or in the hot air handpiece. Usually driven by a brushed or brushless DC motor and associated speed controller.
4. Conventional piston compressor driven by a DC or AC motor - this compressor is relatively complex and expensive to manufacture but has been seen in high end hot air rework stations. In its simplified form it is still seen in a motor driven diaphragm variant for vacuum pickup tools.
5. Regenerative Compressor - Used in the JBC JT6040. Driven by a powerful 240V universal motor and ac speed controller
6. Multi stage turbine compressor ? - Believed to be used in the early JBC JT6040. Driven by a DC brushed Motor and dc speed controller
So why am I listing these different air pump/compressor/blower types ? Well I have discovered something interesting about their differences besides mechanical design. The diaphragm, motor driven piston and Regenerative compressors can produce decent air pressure at low air flow rates. The simple turbine and centrifugal ‘blowers’ are not normally capable of producing much air pressure at low air flow rates. Why does this matter ? Well in the case of a hot air rework station you do not want very high air flow that blows components off of the PCB yet you do want air pressure that is maintained even with highly restrictive small cross section air output nozzles.
So in theory, the diaphragm, motor driven piston and regenerative air compressors are ideal for hot air rework with different output nozzle sizes and lower air flow rates (below 40 l/m) but the centrifugal and turbine blowers are most unsuited to that deployment ! The centrifugal and turbine blowers are best suited to applications that require high volumes of air movement with little variance in back pressure through changes in output port cross sectional area. They are not normally considered appropriate to a interchangeable output nozzle size application as back pressure becomes an issue and these pumps are incapable of producing much pressure, especially at low air flow rates.
So with the above in mind, why are we seeing centrifugal and simple turbine type air blowers in hot air rework stations that, by their very nature present the blower with a less than ideal variable output port size, back pressure and a need for low flow rates ! I am surprised that the blowers even work properly. I can understand a centrifugal or turbine blower used in a hot air paint stripper, heat shrink gun or hair dryer as the output port size is reasonably large, but using such a blower for SMT rework just seems illogical if the air pump/blower manufacturers guidance is to be believed.
We all know that some cheap ‘in handle’ hot air stations that use a centrifugal blower do not shower themselves in glory for various reasons but we are seeing centrifugal air blowers in the recent high power Quick and even JBC hot air stations. For me this is an interesting development. Are those manufacturers just using ‘oversize’ blowers to overcome the pressure issues or is there another clever ‘trick’ being used I wonder ?
I am no expert on air pumps or compressors so welcome others thoughts on the matter. For information, I purchased a pair of very nice AMETEK Microjammer 3” BLDC powered blowers that are great quality. I was thinking of using them in my diaphragm pump based hot air stations, in a similar way that Quick use such a blower. Having read the AMETEK data sheets carefully and their guidance on such blowers..... they seem totally unsuited to the hot air station application. They are recommended for 700l/m unobstructed air flow, not <40l/m highly obstructed output air flow :( Nice blowers though :-+
I will add some information on Air pump/blower selection from AMETEK shortly to this thread. I hope the topic is of interest to others as I have never really bothered to study air pump/blower technology, beyond using cooling fans and seeing various air pumps in equipment.
Fraser
wraper:
--- Quote ---So in theory, the diaphragm, motor driven piston and regenerative air compressors are ideal for hot air rework with different output nozzle sizes and lower air flow rates (below 40 l/m)
--- End quote ---
In practice they are the worst type you can get. They produce highly fluctuating airflow. The result is that even with low airflow it's very easy to knock off components from PCB.
Fraser:
The JBC JT6040 regenerative air pump. Low flow rate but relatively high pressure capability
Fraser:
JBC early JT6040 possible multi stage turbine pump.
Fraser:
The Pace 'turbine' pump
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